Driving circuit for multiple cold cathode fluorescent lamps backlight applications

ABSTRACT

The present invention provides a driving circuit for driving a light source. The driving circuit includes a serial-arranged transformers system having multiple primary windings and secondary windings; a first switch conducting current though a first path and a second switch conducting current though a second path. The first path is a first set of primary windings connected in series and the second path is a second set of primary windings connected in series and the first set of primary transformer windings and the second set of primary transformer windings form the dual primary windings of the transformers respectively. Based on the conduction of each switch, a DC voltage source supplies power to the primary windings of the transformers, which in turn powers on the light source connected to the secondary windings of the transformers.

CROSS REFERENCE TO RELATED APPLICATION

The subject application is a continuation-in-part application of U.S.patent application Ser. No. 10/414,374, entitled “Power Supply for AnLCD PANEL,” filed on Apr. 15, 2003, now U.S. Pat. No. 6,936,975published on Aug. 30, 2005 the teachings of which are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to a driving circuit, and morespecifically, to a circuit for driving light sources.

BACKGROUND OF THE INVENTION

Both the LCD monitor and LCD TV apparatus use the cold cathodefluorescent lamp as a backlight because this lamp has the bestillumination efficiency. Therefore, large-size LCD panels usuallycontain multiple cold cathode fluorescent lamps. There are variousmethods of implementing a DC/AC inverter to drive the multiple coldcathode fluorescent lamps. Topologies such as half-bridge, full-bridge,and push-pull circuits are examples.

FIG. 1 shows a schematic drawing of a conventional center-tap primarywindings push-pull topology used to drive cold cathode fluorescent lampsin accordance with the prior art. DC power 110 provides DC power to thepush-pull circuit. DC power 110 is connected to the primary windings810–814 of the transformers 310–314 which are connected in parallel.Each secondary winding 710–714 of the transformers 310–314 is coupled toa cold cathode fluorescent lamp circuit 410–414. There is a center-tapon each primary winding 810–814. Two power switches 212 and 214 arecoupled to each primary winding 810–814. However, conventional push-pullcircuit is limited due to undesired voltage spikes which are caused bythe leakage-inductance energy of the transformer at the power switcheswhen they are turned off. In addition, for a high-voltage application,the transformer primary winding needs more turns than those inbattery-input applications, which will increase the size and the cost ofthe transformer. And the transformer winding ratio of FIG. 1 is:Ratio≈VOUTrms/VIN  (1)Where VOUTrms is the maximum output voltage and VIN is the minimum inputvoltage of the transformer. The voltage spike across the switch isusually suppressed by a snubbed circuit to absorb the leakage energy.This passive implementation reduces the power conversion efficiency andincreases the system cost with additional parts.

FIG. 2 shows another schematic drawing of a conventional half-bridgetopology used to drive cold cathode fluorescent lamps in accordance withthe prior art. DC power 110 provides DC power to the half-bridgecircuit. Two switches 212 and 214 are coupled to each primary winding810–814 of the transformers 310–314. Each secondary winding 710–714 ofthe transformers 310–314 is coupled to a cold cathode fluorescent lampcircuit 410–414. However, for the bridge-type (either half-bridge orfull-bridge) circuitry, a level shifter circuit and a high-side driveris needed for the power transistor connected to the input voltagesource. For high-voltage applications, this will increase the circuitcost significantly and suffers reliability of switching high-voltagesignals.

SUMMARY OF THE INVENTION

A driving circuit for driving a light is disclosed. The driving circuitincludes a transformer which has multiple primary windings and asecondary winding. A first switch conducts current though a first set ofprimary windings connected in series. A second switch conducts currentthough a second set of primary windings connected in series. The firstset of primary transformer windings and the second set of primarytransformer windings form dual primary windings of the transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated and better understood byreferencing the following detailed description, when taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of a conventional center-tap primarywindings push-pull driving system in accordance with the prior art.

FIG. 2 is another circuit diagram of a conventional half-bridge drivingsystem in accordance with the prior art.

FIG. 3A is an inverter topology for an LCD panel according to oneexemplary embodiment of the present invention.

FIG. 3B is an inverter topology for an LCD panel according to anotherexemplary embodiment of the present invention.

FIG. 4 is a circuit diagram of the transformers and LCD panel of theinverter topology according to the present invention.

FIG. 5 is another circuit diagram of the transformers and LCD panel ofthe inverter topology according to the present invention.

FIG. 6 is a detailed circuit diagram of the LCD power supply system ofthe present invention.

FIG. 7 is another circuit diagram of the transformers and LCD panel ofthe inverter topology according to the present invention.

FIG. 8 depicts another driving topology utilizing two controllers andtwo inverter circuits per CCFL.

FIG. 9 is a circuit diagram of a push-pull driving system for drivingtwo CCFLs in accordance with the present invention.

FIG. 10 is a circuit diagram of a push-pull driving system for driving NCCFLs in accordance with the present invention.

FIG. 11 is a diagram of a block diagram of an examplary display systemutilizing the driving circuit of the present invention.

FIG. 12 is a flow chart of a method for driving a light source inaccordance with the present invention.

DETAILED DESCRIPTION

Typically, there are multiple CCFLs in LCDTV applications to providesufficient brightness on the LCD screen, for example, 4 to 32 CCFLsdepending on the size of the LCD panel. In one aspect of the presentinvention the primary winding of the transformers are coupled in seriesin the power conversion process.

FIG. 3A is an inverter topology 50 for an LCD panel according to oneexemplary embodiment of the present invention. In this exemplaryembodiment, each primary side of the transformers T1 and T2 is connectedin series. Therefore, each primary side sees half of the input voltageacross the winding. In half-bridge applications as depicted in FIG. 3A,placing the primary side of the transformers in series reduces toone-fourth of the input voltage across each winding, and the voltagestress reduced to 1/(2N) of the input voltage when applied to ahalf-bridge application (where N is the number of transformers coupledin series).

FIG. 3B depicts a class D inverter topology, having similar advantagesas set forth above with respect to FIG. 3A since the primary side of thetransformers are coupled in series.

FIG. 4 is a circuit diagram 52 of the transformers and LCD panel of theinverter topology according to the present invention. In this figure,the concept is extended to power four CCFL lamps by coupling fourprimary sides T1, T2, T3, and T4 in series between points A and B ofFIG. 3. Likewise, this topology 52′ is extended to N lamps in FIG. 5which depicts N CCFLs powered by N transformers.

Since each of the primary winding is connected in series, the currentflowing through each transformer primary side is identical during theturn-on, turn-off of the switched network (i.e., the switches of thehalf bridge, full bridge or Class D circuits). The switched network isconnected to point “A” and “B” in FIGS. 3, 4 and 5. This configurationfurther improves the current balance in the secondary side of eachtransformer driving the CCFLs.

FIG. 6 depicts a detailed circuit diagram of an exemplary LCD powersupply system 100 of the present invention. This power supply includesan inverter controller 52 that drives two switches 54 and 56 in a halfbridge circuit, as described in FIG. 3 above. The inverter controller 52includes voltage and current feedback to control the energy of the CCFLscoupled to the circuit. Each CCFL is driven by a primary sidetransformer that is coupled in series as shown (i.e., T1, T2 . . .T(n−1), Tn, Tx; where n represents an even number of lamps, and xrepresents an odd number of lamps) according to the principles anddescription set forth above.

Current feedback is developed with feedback circuitry 60 which isderived from lamps 1 and 2 in the circuit as shown. The exemplarycurrent feedback circuit 60 includes an opto-coupler 62 and a regulator64. The regulator amplifies the current feedback signal Cfb and theopto-coupler 62 sends the feedback information to the controller 52.Similarly, voltage feedback information is developed with voltagefeedback circuitry 70. In this exemplary embodiment, voltage feedbackinformation is taken from each lamp in the circuit to generate a voltagefeedback signal Vb.

The detailed circuit of FIG. 6 also includes other circuitry notdirectly related to the aspects of the present invention. For example, aPWM controller 58 may be provided to generate DC power supply signals(e.g., 12V and 5V) for other components (e.g., memory, microprocessor,etc.) associated with an LCD display. Likewise, the PFC stage 12 mayutilize any conventional and/or custom topology to generate a highvoltage DC signal, as described above.

In another aspect, the present invention provides a circuit topology fordriving long CCFL tubes. The size of the CCFL tubes in LCDTVapplications is usually longer than those in LCD monitor in portableequipment. Driving longer CCFL becomes more difficult. For example, anylamp longer than approximate 60 cm conventional driving methods, ahigh-frequency and high-voltage (normally in the range of 1000V rms) isapplied to the CCFL while one side of the CCFL has a potential nearchassis ground. Due to the leakage current path between the CCFL and thechassis, these driving methods usually encounter a darkness effect onone side of the CCFL. Long lamp may mean 75–80 cm or longer, and isgenerally defined as lamps having a leakage capacitance such that itaffects electron migration between the electrodes of the lamp.

To remedy the difficulty, a differential driving technique is providedby the present invention. As illustrated in FIG. 7, a long lamp can bedriven with two transformers where the phase polarities of thetransformers are opposite. In FIG. 7, CCFL1 is driven by the positiveside of the secondary of T1 and the negative side of the secondary of T2(the positive negative are represented in one half cycle of thesinusoidal power developed by the transformer). The center of CCFL1 isvirtually positioned at zero potential. Each transformer delivers, forexample 500V rms where the voltage stress and mechanical spacing forsafety requirement is lower.

In yet another aspect, the driving techniques may be modified as shownin FIG. 8. FIG. 8 depicts a driving topology 200 utilizing twocontrollers 202 and 204 and two inverter circuits 206 and 208 per CCFL.The inverter circuits are coupled together using synchronization signal210 so that the controllers control their respective inverter circuitsto generate sinusoids that are approximately 180 degrees out of phase,as shown. This ensures that the lamp receives full power from eachinverter during each half cycle without cancellation of the powersignals. Of course, this topology can include voltage and or currentfeedback to control the energy delivered to the lamp.

The inverter controllers of the present invention may be conventionalinverter controllers which may include dimming circuitry (e.g., burstmode, analog, and/or phase) to adjust the energy delivered to the lamps.Inverter controllers capable of controlling. half bridge, full bridge,Class D and/or other inverter topologies are well known in the art, andall are deemed equivalent to the present invention. For example, U.S.Pat. Nos. 6,259,615 and 5,615,093, hereby incorporated by reference,each disclose inverter controllers for full bridge and half bridgeinverter circuits, respectively. The inverter controllers may also beimplemented by, for example Part Nos. OZ960, OZ961, OZ965, OZ970, OZ971,OZ972, or OZ9RR, manufactured by O2Micro International Limited.

Also, the figures depict an LCD panel that includes circuitry togenerate a voltage and/or current feedback signal indicative of thevoltage and/or current conditions at the lamp load. The invertercontroller depicted herein is also adapted to receive this feedbackinformation to adjust the voltage and/or current supplied to the lamploads. In the exemplary embodiments, current feedback can be generatedfrom a single lamp in the two lamp panel of FIGS. 3A and 3B, or from twolamps in an N lamp panel as shown in FIGS. 4, 5 and 6. In FIG. 7, thecurrent feedback control signals are generated from the portion of thetransformer secondary side that is not coupled to the lamp. In thismanner, each half cycle of current to the lamp is monitored. Likewise,voltage feedback control signals can be generated in a manner understoodin the art.

In the exemplary embodiments, the transformers are coupled to the powersupply as controlled by the inverter controller. The inverter controllergenerates a high voltage AC signal (square wave) from the high voltageDC signal source. In turn, the transformers produce high voltagesinusoidal power from the high voltage AC signal to power the lamps. Ofcourse, the present invention can use a low voltage DC power source, inwhich case the transformers will be adapted to step up the voltage to anappropriate level to power the lamps. Those skilled in the art willrecognize numerous modifications to the present invention, all of whichare deemed within the spirit and scope of the present invention only aslimited by the claims.

Turning to FIG. 9, a circuit diagram of a push-pull driving system 10 inaccordance with the present invention is depicted. More specifically,the system 10 is an exemplary driving system for driving two coldcathode fluorescent lamps. The exemplary embodiments will be describedherein with reference to cold cathode fluorescent lamps. However, thepresent invention is applicable to any type of load.

The system 10 generally includes a DC voltage source 110, two powertransistors 212 and 214, two transformers 310, 312 and two cold cathodefluorescent lamps 410, 412. DC voltage source 110 can be a DC/DCconverter or can be rectified from an AC voltage source. Transformer 310has a core 510, and a corresponding dual primary winding 810 and asecondary winding 710. Similarly, transformer 312 has a core 512, and acorresponding dual primary winding 812 and a secondary winding 712. Oneset of the primary windings (820 and 822) are coupled in series and theother set of the primary windings (830 and 832) are coupled in series,which comprise the serial-arranged dual primary windings 810 and 812respectively, as shown in FIG. 9. Such that transformers 310 and 312have their dual primary windings 810 and 812 connected in series. Andthe secondary windings (710 and 712) of both transformers are connectedto lamps 410 and 412, respectively. Power switches 212 and 214 describedherein are N type MOSFETs. The source of MOSFET 212 is connected toground, the gate of MOSFET 212 is connected to PWM signal 102 and thedrain of MOSFET 212 is connected to the primary winding 820 oftransformer 310. Similarly, the source of MOSFET 214 is connected toground, the gate of MOSFET 214 is connected to PWM signal 104 and thedrain of MOSFET 214 is connected to the primary winding 832 oftransformer 312. Any other type of power transistor such as bipolarjunction transistor, insulated gate bipolar junction transistor andother equivalent transistors can be used as the switch of the presentinvention. The primary winding voltage can be derived from DC voltagesource 110.

Signal 102 and signal 104 are pulse-width modulation signals provided bya pulse-width modulation generator 500. Signal 102 is coupled to MOSFET212 and signal 104 is coupled to MOSFET 214. PWM signal 102 has a 180degree phase shift to PWM signal 104. The duty cycles of both PWM signal102 and PWM signal 104 are less than 50%. MOSFETs 212 and 214 are turnedon by the high level of the PWM signal and turned off by the low levelof the PWM signal. Because of the 180 degree phase shift between PWMsignal 102 and PWM signal 104, MOSFET 212 and MOSFET 214 will beconducted alternately. In operation, when PWM signal 102 is HIGH and PWMsignal 104 is LOW, MOSFET 212 will be turned on (ON) while MOSFET 214will be turned off (OFF). The current is flowing through the first pathfrom DC voltage source 110 to MOSFET 212. Then PWM signal 102 is LOW andPWM signal 104 is HIGH, thus MOSFET 214 is ON and MOSFET 212 is OFF. Asa result, the current is flowing through the second path from DC voltagesource 110 to MOSFET 214. By alternately conducting MOSFET 212 andMOSFET 214, an AC current will be generated in the dual primary windings810 and 812 of the transformers 310 and 312. Thus the cold cathodefluorescent lamps connected to the secondary windings 710 and 712 of thetransformers 310 and 312 are powered.

The transformer winding ratio of present invention is given by theequation:Ratio≈VOUTrms*N/VIN  (2)Where N is the number of the transformers; VOUTrms is the maximum outputvoltage and VIN is the minimum input voltage of the transformer. Thetransformer winding ratio of present invention is ranged from 10 to 100.In the preferred embodiment, there are two transformers. Then equation(2) yields in:Ratio≈VOUTrms*2/VIN  (3)From equation (1), Ratio increases in proportion to the increase of N.Since Ratio≈Nout/Nin, where Nout is the secondary winding turns and Ninis the primary winding turns of the transformer, thus the primarywinding turns is decreased compared to the prior art. Therefore, theleakage inductance of the primary winding is decreased. As a result, thevoltage spike caused by the resonance of the leakage inductance and theparasitical capacitance of the power transistor will be reduced.

The driving topology described above can be extended to multi-lampenvironments. FIG. 10 is an exemplary push-pull driving system 10′ fordriving N cold cathode fluorescent lamps. In an N lamp system, there areN cold cathode fluorescent lamps and respective N secondary windings. Asshown in FIG. 10, the first cold cathode fluorescent lamp is labeled as410, the second cold cathode fluorescent lamp is labeled as 412, and theN cold cathode fluorescent lamp is labeled as 414. Accordingly, N cores510–514 are provided. One set of the primary windings (820–824) arecoupled in series and the other set of the primary windings (830–834)are coupled in series, which comprise the serial-arranged dual primarywindings 810–814 respectively. Some components which are similar tothose in FIG. 9 are labeled similarly and will not be detailed describedherein for clarity. Likewise, the source of MOSFET 212 is connected toground, the gate of MOSFET 212 is connected to PWM signal 102 and thedrain of MOSFET 212 is connected to the primary winding 820 oftransformer 310, which has core 510. And the source of MOSFET 214 isconnected to ground, the gate of MOSFET 214 is connected to PWM signal104 and the drain of MOSFET 214 is connected to the primary side 834 ofthe N transformer 314, which has core 514. The operation of FIG. 10 issimilar to that of FIG. 9 and will not be repeatedly described herein.By alternately conducting MOSFET 212 and MOSFET 214, an AC current willbe generated in the dual primary windings 810–814 of N transformers310–314, which have separate cores 510–514 respectively. Therefore Ncold cathode fluorescent lamps 410–414 connected to the secondarywindings 710–714 of the N transformers 310–314 are illuminated.

In FIG. 9 and FIG. 10, we are discussing multiple cores applications. InFIG. 9, there are two cores 510 and 512, while in FIG. 10 there are Ncores 510–514. Alternatively, the present invention can be implementedwith a single core. For example, in a single core implementation, withreference to FIG. 9, it also can be a single core with two secondarywindings 710 and 712 coupled to CCFL 410 and 412 respectively, and withone set of the primary windings (820 and 830) coupled in series and theother set of primary windings (822 and 832) coupled in series, whichcomprise the serial-arranged dual primary windings 810 and 812respectively. All the other elements are the same as those of FIG. 9,except the single core instead of two cores. Similarly, in FIG. 10, Ncores 510–514 can be replaced by a single core. In this situation, itcan be a single core with N secondary windings 710–714 coupled to NCCFLs 410–412 respectively, and with one set of primary windings(820–824) coupled in series and the other set of primary windings(830–834) coupled in series, which comprise the serial-arranged dualprimary windings 810–814 respectively.

Turning to FIG. 11, it depicts a block diagram of a display systemutilizing the driving circuit of the present invention. Morespecifically, FIG. 11 depicts a LCD panel 560 of LCD monitor system 580utilizing the driving circuit 10′ of the present invention. As a generalmatter, the LCD panel 560 of the LCD monitor system 580 includesmultiple cold cathode fluorescent lamps 540 and driving circuit 10′accordance with the present invention. Once the LCD monitor system 580powered on, the DC voltage source 110 will deliver power to drivingcircuit 10′. The detailed operation of the present embodiment is similarto the preferred embodiment of FIG. 9 and FIG. 10, and will not be fullydescribed herein. Thus the cold cathode fluorescent lamps 540 arepowered and will provide backlight to the LCD panel 560. The presentinvention has utility in applications which utilize cold cathodefluorescent lamp technology, for example, display systems found in LCDmonitor, LCD TV etc., as described herein; although other utilities arecontemplated herein.

FIG. 12 shows a flow chart 600 of a method for driving a light source inaccordance with the present invention. As shown in the figure, in step602, PWM generator 500 provides PWM signal 102 and 104 to switch 212 and214 respectively. Assume PWM signal 102 is HIGH and PWM signal 104 isLOW since PWM signal 102 has a 180 degree phase shift to PWM signal 104.Thus in step 604, switch 212 is ON and switch 214 is OFF. So the currentis flowing through the first path in step 606. In step 608, PWM signal102 is LOW and PWM signal 104 is HIGH. Similarly, the current is flowingthrough the second path in step 610. Then PWM signal 102 is HIGH and PWMsignal 104 is LOW again and the above process is repeating. As a result,the current is flowing through the first path and the second pathalternately along with the conductance of switch 212 and switch 214respectively. Therefore, an AC current is generated in the primarywindings and the secondary windings in step 612, which in turn powersthe cold cathode fluorescent in step 614.

The driving circuit topology of the present invention provides thefollowing advantages. First, this driving circuit needs less primarywinding turns compared to prior art. Second, the voltage spike on powerswitch caused by the resonance of the leakage inductance and theparasitical capacitance of the power transistor is thus reduced. Third,since less primary winding turns are needed, the circuit structure issmaller in size and significant cost savings is achieved in the presentinvention over prior art techniques. The present invention provides acost effective and better-performance derived push-pull circuitry for ahigh-voltage DC/AC backlight inverter applications.

The foregoing descriptions of the preferred embodiment of the presentinvention are an illustration of the present invention rather than alimitation thereof. It is intended to cover various modifications andsimilar arrangements included within the spirit and scope of theappended claims. While the preferred embodiments of the invention hasbeen illustrated and described, it will be appreciated that variouschanges can be made therein without departing from the spirit and scopeof the invention.

1. A driving circuit for driving a light source comprising: atransformer comprising multiple primary windings and a secondarywinding, wherein said multiple primary windings are coupled to a powersource and said secondary winding is coupled to said light source; afirst switch for conducting current through a first path comprising afirst set of primary windings coupled in series; a second switch forconducting current through a second path comprising a second set ofprimary windings coupled in series, wherein said first set of primarywindings and said second set of primary windings comprise said multipleprimary windings corresponding to said transformer; and a pulse-widthmodulation generator configured to generate a first pulse-widthmodulation signal and a second pulse-width modulation signal to saidfirst switch and second switch respectively, wherein said firstpulse-width modulation signal has a 180 degree phase shift to saidsecond pulse-width modulation signal.
 2. The driving circuit as claimedin claim 1, wherein said driving circuit comprises a plurality oftransformers.
 3. The driving circuit as claimed in claim 2, wherein saidplurality of transformers comprise a plurality of secondary windings. 4.The driving circuit as claimed in claim 3, wherein said multiple primarywindings comprise dual primary windings.
 5. The driving circuit asclaimed in claim 4, wherein said dual primary windings are wound onmultiple cores.
 6. The driving circuit as claimed in claim 4, whereinsaid dual primary windings are wound on a single core.
 7. The drivingcircuit as claimed in claim 5, wherein said secondary windings are woundon said multiple cores.
 8. The driving circuit as claimed in claim 6,wherein said secondary windings are wound on a single core.
 9. Thedriving circuit as claimed in claim 4, wherein said dual primary windingis coupled to said first switch and said second switch.
 10. The drivingcircuit as claimed in claim 1, wherein the winding ratio of saidtransformer ranges from 10 to
 100. 11. The driving circuit as claimed inclaim 1, wherein said first switch is selected from a transistor groupcomprising metal oxide semiconductor field effect transistor, bipolarjunction transistor, and insulated gate bipolar junction transistor. 12.The driving circuit as claimed in claim 1, wherein said second switch isselected from a transistor group comprising of metal oxide semiconductorfield effect transistor, bipolar junction transistor, and insulated gatebipolar junction transistor.
 13. A method for driving a light source,said method comprising: switching a first switch to conduct currentthrough a first path comprising a first set of primary windings coupledin series in a transformer; switching a second switch to conduct currentthrough a second path comprising a second set of primary windingscoupled in series in said transformer, wherein said first set of primarywindings and said second set of primary windings comprise multipleprimary windings corresponding to said transformer; and using a firstpulse-width modulation signal and a second pulse-width modulation signalto switch said first switch and second switch respectively, wherein saidfirst pulse-width modulation signal has a 180 degree phase shift to saidsecond pulse-width modulation signal.
 14. The driving circuit as claimedin claim 13, wherein said first set of primary windings and said secondset of primary windings comprise multiple primary windings correspondingto a plurality of transformers having a plurality of secondary windingscoupled to said light source.
 15. The method as claimed in claim 14,wherein said multiple primary windings comprise dual primary windings.16. The method as claimed in claim 13 further comprises the step ofselectively switching said first switch and said second switchalternately.
 17. The method as claimed in claim 16, wherein said currentflows through said first path to said first switch if said first switchis turned on.
 18. The method as claimed in claim 16, wherein saidcurrent flows through said second path to said second switch if saidsecond switch is turned on.
 19. The method as claimed in claim 15,wherein said dual primary windings are on multiple cores respectively.20. The method as claimed in claim 15, wherein said dual primarywindings are on a single core.
 21. The method as claimed in claim 19,wherein said secondary windings are on said multiple cores respectively.22. The method as claimed in claim 20, said secondary windings are onsaid single core.
 23. The method as claimed in claim 13, wherein saidfirst switch is selected from the transistor group comprising of metaloxide semiconductor field effect transistor, bipolar junctiontransistor, and insulated gate bipolar junction transistor.
 24. Themethod as claimed in claim 13, wherein said second switch is selectedfrom the transistor group comprising of metal oxide semiconductor fieldeffect transistor, bipolar junction transistor, and insulated gatebipolar junction transistor.
 25. A display system, comprising: aplurality of light sources; and a driving circuit for driving saidplurality of light sources, said driving circuit comprising: atransformer comprising multiple primary windings and a secondarywinding; a first switch for conducting current through a first pathcomprising a first set of primary windings coupled in series; a secondswitch for conducting current through a second path comprising a secondset of primary windings coupled in series, wherein said first set ofprimary windings and said second set of primary windings comprise saidmultiple primary windings corresponding to said transformer; and apulse-width modulation generator configured to generate a firstpulse-width modulation signal and a second pulse-width modulation signalto said first switch and second switch respectively, wherein said firstpulse-width modulation signal has a 180 degree phase shift to saidsecond pulse-width modulation signal.
 26. The display system as claimedin claim 25, wherein said driving circuit comprises a plurality oftransformers.
 27. The display system as claimed in claim 26, whereinsaid plurality of transformers comprise a plurality of secondarywindings.
 28. The display system as claimed in claim 27, wherein saidmultiple primary windings comprise dual primary windings.
 29. Thedisplay system as claimed in claim 28, wherein said dual primarywindings are on multiple cores respectively.
 30. The display system asclaimed in claim 28, wherein said dual primary windings are on a singlecore.
 31. The display system as claimed in claim 29, wherein saidsecondary windings are on said multiple cores respectively.
 32. Thedisplay system as claimed in claim 30, wherein said secondary windingsare on said single core.
 33. The display system as claimed in claim 28,wherein said dual primary winding is coupled to said first switch andsaid second switch.
 34. The display system as claimed in claim 25,wherein the winding ratio of said transformer ranges from 10 to 100.